The present invention relates generally to diverter valves and, more particularly, to slide plate diverter valves of the type utilized in directing the flow of viscous materials such as molten thermoplastic polymers under relatively high pressures.
In many manufacturing operations involving the formation of products from molten thermoplastic material, e.g., plastic extrusion, injection molding, and blow molding operations, it is desirable, if not essential, that the operation be carried out on a continuous flow basis. Additionally, it is common practice in such operations to filter impurities from the thermoplastic material by passage through a suitable filtration unit. Necessarily, such filtration units require periodic cleaning and, accordingly, to accommodate such cleaning and also permit the manufacturing operation to proceed on a continuous basis, dual filters are typically utilized with a diverter valve situated in the flow line to selectively direct the fluid material through one of the filters while the other is inactive for periodic cleaning and maintenance and otherwise to serve as a back-up to the operating filter.
It is therefore critical in such manufacturing operations that the diverter valve be capable of functioning reliably over extended periods of time, which objective is complicated by the severe temperature and pressure conditions under which such valves often operate. In particular, molten thermoplastic materials often have temperatures in excess of 400 degrees Fahrenheit and even as high as 600 degrees Fahrenheit or more and often flow at pressures in excess of 1000 PSI up to 5000 PSI. Such materials may also be corrosive in nature. Other requirements placed on such valves are that their configuration must be such that the fluid flow characteristics do not differ between the alternate operating dispositions of the valve and, especially, the flow passageways through the valve must be configured to avoid any stagnation in the fluid flow which could cause certain thermoplastic materials to undergo changes in their physical or chemical character.
One such diverter valve which addresses these objectives in commercial practice is a diverter valve of the slide plate type disclosed in Blanchard U.S. Pat. No. 4,334,552. Basically, this valve provides a manifold defining inlet and outlet fluid flow passages opening at a sealing face thereof through respective ports, a slide plate defining a fluid flow passageway at a sealing face thereof in facing surface contact to the manifold sealing face, and a backing plate disposed at the opposite side of the slide plate to sandwich it between the backing plate and the manifold, with a plurality of bolts connecting the manifold and backing plate for clamping them together to hold the respective sealing faces of the manifold and the slide plate in metal-to-metal surface contact with substantially zero clearance therebetween. The Blanchard patent teaches that the bolts effectively function as a spring to enable the valve to absorb and accommodate fluctuations in fluid pressure.
Disadvantageously, it has been observed in practice that the valve of the Blanchard patent tends to suffer sticking of the slide plate making it difficult to move the slide plate smoothly and position it accurately, which is believed to be due to the clamping forces exerted by the valve on the slide plate. It has also been observed in practice that, during changeovers between the opposite limit positions of the slide plate, the valve will produce startlingly loud reports. While the cause of such reports has not been definitely determined, it is believed that the reports result from the fact that the bolts permit a limited but sufficient amount of relative lateral movement of the backing plate with respect to the manifold that, upon actuation of the slide plate, the backing plate tends to move with the slide plate until resisted by the bolts and then to snap back into the original disposition of the backing plate. Regardless of the cause of such reports, the reports cause users of such slide plate diverter valves to be characteristically fearful of the possibility of valve failure and, as a result, it is quite common for users to severely over-torque the bolts in an attempt to better secure the valve but, disadvantageously, such steps may actually exacerbate the problems with sticking and inaccurate position of the slide plate as well as weakening the valve and increasing the risk of failure.